I don't know the full datails but this particular series - the albums I mentioned above will only be the first batch of many - is dedicated to the collecting work that PK did (though Shirley has argued the inclusion of those of Bob Copper). There will be recordings that Peter made in the north of Ireland, but as far as I know, he did not record Irish Travellers - so these will be missing.

On the other hand, there will be the first decent release (forgetting the scrappy Folktrax ones) of Carolyn Hughes. Not the easiest of tastes for the newcomer to acquire, but when you do, you realise that she is a major, major singer.

The Scots Travellers? Well, Peter worked with Alan Lomax 1951-3 and between them they recorded masses of stuff from the finest singers, but a lot of those recordings - Jeannie Robertson, Davy Stewart, Jimmy MacBeath came out not so many years ago in the Lomax/Rounder series.
On my Folktrax FTX-058 - TRAMPS AND HAWKERS JIMMY MacBEATH -1- it says:-

Recorded 1951-3. Edited by Peter Kennedy and first published by Folktrax 1975.

So "edited" by PK but recorded by...? possibly Lomax.

As I said, I don't know the full details of the new series - even if these have been finalised yet - but Steve Roud is compiling ballad albums for this series and when I interrupted a meeting of Mssrs Roud, Engle, Hall and Ms Collins, they were listening to Jeannie Robertson.